1. Field of the Invention
Aspects of the present invention relate to a thin film transistor (TFT), a method of fabricating the same, and an organic light emitting diode (OLED) display device including the TFT. More particularly, aspects of the present invention relate to a TFT whose electrical properties are improved by controlling the concentration of a metal catalyst of a crystallized semiconductor layer, a method of fabricating the TFT, and an OLED display device including the TFT.
2. Description of the Related Art
In general, a polycrystalline silicon (poly-Si) layer is widely used as a semiconductor layer for a thin film transistor (TFT) because polycrystalline silicon has high field-effect mobility, is suitable for a high-speed operating circuit, and may be used to configure a complementary-metal-oxide-semiconductor (CMOS) circuit. A TFT using a poly-Si layer may function as an active device of an active-matrix liquid crystal display (AMLCD) or a switching device or a driving device of an organic light emitting diode (OLED). Methods of crystallizing an amorphous silicon (a-Si) layer into a poly-Si layer may include solid phase crystallization (SPC), excimer laser annealing (ELA), metal induced crystallization (MIC), and metal induced lateral crystallization (MILC).
Specifically, the SPC method includes annealing an a-Si layer for several hours to several tens of hours at temperatures below 700° C. (Temperatures above 700° are not used in annealing an a-Si layer to be used in a TFT for a display device because the glass substrate used in the TFT becomes deformed at such temperatures). The ELA method includes partially heating an a-Si layer to a high temperature in a short amount of time by irradiating excimer laser beams onto the a-Si layer. The MIC method includes bringing a metal, such as nickel (Ni), palladium (Pd), gold (Au), or aluminum (Al), into contact with an a-Si layer or injecting the metal into the a-Si layer to induce a phase transition from the a-Si layer to a poly-Si layer. The MILC method includes sequentially inducing the crystallization of an a-Si layer while laterally diffusing silicide formed by a reaction of a metal with silicon.
However, since the SPC method involves annealing a substrate at a high temperature for a long period of time, great damage may be done to the substrate, even at temperatures below 700° C. The ELA method not only requires expensive laser apparatuses but also may result in protrusions being formed in the surface of the poly-Si layer, which may degrade interface characteristics between a semiconductor layer and a gate insulating layer.
Recently, a vast amount of research has been being conducted on methods of crystallizing an a-Si layer using a metal catalyst because the a-Si layer may be crystallized at a lower temperature for a shorter amount of time than in an SPC method. Typical methods of crystallizing an a-Si layer using a metal catalyst are the MIC method, the MILC method, and a super grain silicon (SGS) method.
A leakage current significantly affects the characteristics of a TFT. In particular, a metal catalyst may remain in a channel region of a semiconductor layer that is crystallized using the metal catalyst, thereby increasing the leakage current. Accordingly, if the concentration of the metal catalyst remaining in the channel region is not controlled, the leakage current of the TFT may increase, thereby degrading its electrical properties.